Soft contact lenses can be made from hydrophilic polymers also known as hydrogels by many processes, for example, by molding, spin casting or lathe cutting.
During the initial steps of manufacturing hydrogel lenses by lathe cutting, the hydrogel is maintained in a dry state so that it may be manipulated on the lathe to cut the desired optical surfaces. It is then polished, removed from the lathe, hydrated, inspected and packaged. Spin cast lenses can be made by subjecting a liquid monomer to a centrifical force in a mold which has the same shape as the desired optical surfaces of the lens. As the mold spins, the monomer is cured to form the lens. The lens is then typically polished in the mold, hydrated from the mold, inspected and packaged. Other lenses may be molded from liquid monomer and catalyst confined in a space between two mold halves which control the shape of the lens. The monomer is cured, the mold halves separated and the lens is hydrated, inspected and packaged. The hydration step in all of these processes can be time consuming, difficult to control and somewhat unpredictable so that the manufacturing process can be inefficient and sometimes expensive.
In carrying out many of the processes for making contact lenses, significant amounts of manual handling of the lens may be required. Handling the lens in the dry state can dirty or scratch the lens. Handling the lens in the delicate wet state can cause tears and other imperfections. It would be particularly useful to have a lens manufacturing process which would minimize the handling of the lens.
When removing a lens from a final package, a user often has difficulty orienting the lens properly for placement on the cornea of the eye. Sometimes the lens inverts or rolls over so that the user can inadvertently put the wrong surface of the lens facing the cornea. It would be desirable to have a process in which one could control the orientation of the lens throughout the process and uniformly place the lens in the package so that it could be consistently removed by the user in the proper orientation for placement on the cornea. It would also be useful to have a package specially designed to maintain the orientation of the lens during storage and shipping.
The Assignee of the present invention molds its contact lenses in two-part molds like those shown in U.S. Pat. Nos. 4,565,348 and 4,640,489. Liquid monomer is placed in a concave mold surface and then covered with a lid and cured, for example, by ultraviolet light. During polymerization, particularly of hydrogels, the lens tends to shrink. To reduce shrinkage, the monomer is polymerized in the presence of an inert diluent like boric acid ester as described in U.S. Pat. No. 4,495,313. The inert diluent fills up the spaces in the hydrogel lens during polymerization. The diluent is subsequently exchanged for water during the hydration process. Since the boric acid ester is inert but water soluble, it can be used to fill up the spaces in the hydrogel during polymerization to minimize shrinkage of the lens during polymerization and then exchanged for water to hydrate the lens. This process significantly improves the reliability of the manufacturing process and increases the ability to predict and maintain the dimensions of the lens during processing. The entire disclosure of U.S. Pat. Nos. 4,565,348; 4,640,489 and 4,495,313 are hereby incorporated by reference into the present patent application.
The process of exchanging the diluent for water and hydrating the lens can be very time consuming. The two-part mold is opened or demolded and the lenses are assembled in large groups and placed in a leaching tank for several hours. The leach tank includes heated water, small amounts of surface-active agents (surfactants) and salts. When the lenses are inserted in the leach tank they immediately expand in the presence of water and release from the mold in which they were molded. The boric acid ester diluent hydrolyzes into glycerol and boric acid leaving the water behind in the matrix of the lens to thus exchange diluent for water to partially hydrate the lens.
Salts and a ph buffer are used in the water so that the water placed in the lens has osmolality and ph substantially similar to that of human tears so that the lens will not irritate the eye when it is inserted by the user. If the polymer from which the lens is made has ionic characteristics the buffer neutralizes any ionic species in the lens. That neutralization causes some temporary destabilization of the dimensions of the lens and requires an extended period of time to complete.
The leach solution is then drained and the lenses are transferred to a rinse tank where the removal of diluent and surfactant continues for another extended period of time. The rinse solution is then drained and the lenses are transferred to a large equilibration tank filled with heated water and salts for completion of diluent and surfactant removal and equilibration of the lens for several more hours. The equilibration step entails completion of the neutralization of any ionic species in the polymer from which the lens is made and final hydration to the final water content and final dimensionals. The lens is then removed from the equilibration tank and rinsed in clean saline and transferred for inspection and then packaging.
It would be desirable to have a process for hydrating lenses that would reduce the amount of water, associated chemicals, like surfactants and salts, and the amount of time necessary to complete hydration and control the orientation of the lens.
It would also be useful to control the orientation of the lens during the hydration process so that it could consistently be placed in the package with the correct orientation.